Add support for dividing a run's boss battle progression into sections like "Main Story" and "Endgame" (e.g., post-Elite Four content). This helps players visually distinguish where the main campaign ends and optional/endgame content begins.
## Context
Currently boss battles are displayed as a flat ordered list. In many Pokemon games there's a clear distinction between the main story (up through the Champion) and endgame content (rematches, Battle Frontier, Kanto in GSC/HGSS, etc.). A visual divider would make it easier to track progress through each phase.
## Scope
- **Admin side**: Allow marking boss battles or defining breakpoints that separate progression phases (e.g., "everything after this boss is endgame")
- **Run side**: Render a visual divider/section header between main story and endgame boss battles
- Should support at minimum two sections (main story, endgame), but consider whether the design should be flexible enough for arbitrary sections (e.g., "Kanto" in HGSS)
## Checklist
- [ ] Decide on data model approach (e.g., a `section` field on boss battles, or a separate progression divider entity tied to the version group)
- [ ] Add backend models and migrations
- [ ] Add API support for managing sections/dividers
- [ ] Update admin UI to allow assigning bosses to sections or inserting dividers
- [ ] Update run-side boss progression display to render section headers/dividers
